This invention relates to a screw fastener used with a tapping screw for fastening various kinds of members to e.g. a concrete wall to prevent the tapping screw from loosening.
Various fastening means are known for fastening various kinds of members to concrete, ALC (air bubble lightweight concrete), plaster boards or the like. A suitable fastening means is selected according to the material of the member to be fastened. Among such fastening means, tapping screws have advantages in that they are cheap and that two members can be fastened together fairly easily by using them.
One conventional way to fasten a certain member to concrete was to drive tapping screws into starting holes formed in the concrete. Another way was to embed plugs or anchors in the concrete and drive screws into them. In order to fasten a member to ALC, after embedding a plug or an anchor for use with ALC, screws were driven into them. Otherwise, nails for use with ALC such as leg-spreadable nails were driven into ALC.
For plaster boards, too, it was necessary to embed plugs or anchors for use with plaster boards and drive screws into them.
In driving tapping screws directly into concrete, if the starting hole formed in the concrete by drilling has an inner diameter even slightly larger than the diameter of the threads on the tapping screw, the tapping screw may slip and idle when tightened. If too small, it will become very difficult to drive the screw into concrete.
On the other hand, since ALC and plaster boards are brittle, if tapping screws were driven directly into them, they might be chipped off by the screws. If this happens, the screws would slip and loosen.
When using plugs or anchors, as shown in FIG. 6, a plug 42 or an anchor is inserted in a starting hole 41 formed in a member 51 into which a screw is to be driven. A screw is driven into the plug 42 or anchor and tightened.
The plug 42 has shoulders 43 on the outer periphery thereof to prevent it from coming out of the hole in the member and is formed with an axial bore 44 extending from the top end to an intermediate portion thereof. Further, the plug has slits 45 that extend from the bottom end thereof to its intermediate portion so as to divide the lower part of the plug into a plurality of portions. A tapping screw is inserted into the bore 44 and then driven into the lower part of the plug while radially expanding the plurality of divided portions. The plug is thus tightly pressed against the wall defining the hole. Thus, the plug 42 and the tapping screw fastened to the plug can be held in position so as not to come out of the hole. The anchor is of substantially the same structure as the plug.
With such a conventional method in which plugs 42 or anchors are used, since tapping screws are driven into the plugs 42 or anchors inserted in the starting holes 41 formed in the member 51, the tapping screws never directly come into contact with the member 51. Thus, when the tapping screws are initially driven into the plugs 42 or anchors, the latter are liable to turn together with the screws being driven in. If this happens, the screws cannot be fastened tightly. This tendency is pronounced especially if the member 51 is made of a brittle material.
Furthermore, the use of plugs 42 or anchors will lead to increased cost and longer working time.